Are you using the clipboard brush, by any chance? It doesn't seem to work (possibly because the clibboard is always RGB). Exporting a grayscale-mode image to GBR ought to work. Can you attach the brush here? (either as GBR or in a ZIP)
PS: The Reset button at the end of the Size slider in the Tool options resets the tool brush size to the native size of the brush.
PS: The Reset button at the end of the Size slider in the Tool options resets the tool brush size to the native size of the brush.